Reduce Teleport's glibc version requirements

[webvictim]

Using Ubuntu 18.04 as our buildbox base image means that binaries created by Drone have a minimum glibc version of 2.27. We should build our binaries in a buildbox with a lower glibc version to make them more compatible with older distributions.

Originally posted by @webvictim in #4754 (comment):

Statically-compiled binaries would fix this (#4896) but are untested and have historically introduced a number of other issues with unpredictable DNS resolution when running in Docker/Kubernetes.

One workaround for this which would prevent huge fragmentation of our build matrix while also not needing us to switch to statically-compiled binaries may be to just change the Teleport buildbox to always bootstrap and build Go from source in a container which uses an older glibc version; we do this already for CentOS 6 binaries. We could use CentOS 7 which would give us glibc 2.18.

Here's the list of required glibc versions that I know of so far (run ldd --version to see the required glibc version for a given distro):

Distro	glibc version
CentOS 6	2.12
CentOS 7	2.18
CentOS 8	2.28
Ubuntu 18.04	2.27
Ubuntu 18.10	2.28
Ubuntu 20.04	2.31
Ubuntu 20.10	2.32
Fedora 32	2.31
Synology DSM 6.2.3	2.20
Amazon Linux 2	2.26
NVIDIA Jetson AGX	2.27

[zmb3]

We should get back down to GLIBC 2.18 when @LKozlowski's rustls changes are finalized, which turns out to be good enough since we no longer support CentOS 6.

[webvictim]

This is fantastic - back to one RPM/package for all :)

[jakule]

I think this issue can be closed as we switched our buildbox to CentOS 7 #14062